ublk: add control command of UBLK_U_CMD_GET_FEATURES

Add control command of UBLK_U_CMD_GET_FEATURES for returning driver's
feature set or capability.

This way can simplify userspace for maintaining compatibility because
userspace doesn't need to send command to one device for querying driver
feature set any more. Such as, with the queried feature set, userspace
can choose to use:

- UBLK_CMD_GET_DEV_INFO2 or UBLK_CMD_GET_DEV_INFO,
- UBLK_U_CMD_* or UBLK_CMD_*

Userspace code:
	https://github.com/ming1/ubdsrv/commits/features-cmd



Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230603040601.775227-1-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>